It is common practice in the medical field to employ intravascular catheter systems for clinical monitoring and recording of a patient's arterial pulse waveforms to determine various parameters such as heart rate, stroke volume, cardiac output, duration of systole, and systolic, diastolic, and mean blood pressures. In addition, blood samples for intermittent arterial blood gas analysis purposes may be withdrawn from time to time from the patient by such catheter systems.
The catheter systems in general use at present ordinarily include a conventional catheter for injection into the patient's artery or vein and connected, through a tubing system containing a flow control device, to a source of parenteral liquid such as normal saline solution under pressure. The catheter is also connected, downstream of the flow control device, by a connecting tube to a pressure transducer for converting the physical blood pressure signals into an electrical impulse which is then fed to a recording machine such as an oscilloscope. The aforementioned connecting tube may be provided with a coupling for attachment thereto of a syringe for withdrawing blood samples from the patient from time to time for blood gas analysis.
In order to assure high fidelity clinical monitoring or recording of central arterial pulse waveforms by the use of such intravascular catheter systems, it has been necessary to continuously flush the inserted catheter, during use, with a regulated continuous infusion thereinto of a relatively small flow, e.g., around 3 cc to 7 cc per hour, of a parenteral liquid such as a normal saline solution, in order to prevent occlusion of the intravascular end of the catheter by blood coagulation. Catheter patency is thereby maintained for continuous monitoring or recording of the arterial pulse waveforms over periods of time which may amount to several days. The small amounts of parenteral liquid infused into the patient as a result of such continuous catheter flushing is easily absorbed by the patient's body and is in no way harmful. Continuous catheter flushing systems such as described and utilizing so-called marine-bore capillary tubes as flow resistors and applying the flushing solution under pressure have been employed heretofore, one such system being described in an article appearing on pages 675-678 of the Journal of Thoracic and Cardiovascular Surgery, Volume 57, No. 5, May 1969.
Prior to the insertion of an intravascular catheter such as described above into a patient's artery or vein for clinical monitoring of pulse waveforms, as well as periodically during the progress of such monitoring, it becomes necessary to flush a larger amount of the flushing solution through the catheter in order to prime or quickly fill the catheter tubing system with such solution in the first instance and eliminate any possible air bubbles therefrom, and particularly to ensure that the catheter is and remains completely free of coagulated blood. Various types of continuous catheter flushing systems for selectively providing either a slow or a fast flow rate of the flushing solution have been proposed heretofore. Thus, in U.S. Pat. No. 3,581,733, a saline flushing solution is directed by a stopcock either through a large capillary tube or through a larger separate bypass tube to a channel tubing connected to the catheter, and a second stopcock is connected between the catheter and the channel tubing to enable the quick and complete filling or priming of the tubing system with the flushing solution. Such a system is complicated to set-up, requiring several connections to stopcocks and tubing sections. Moreover, the use of stopcocks in the system results in a decrease of pressure pulse fidelity because of the often present minute leaks in the stopcocks.
Many of the above referred to deficiencies inherent in the earlier continuous catheter flushing system such as described above are overcome by the systems disclosed in U.S. Pat. Nos. 3,675,891 and 4,200,119 wherein a unitary flow control device connected in the tubing system supplying the flushing solution to the catheter is so constructed as to eliminate the use of all stopcocks and itself selectively provide either a capillary restricted flow rate or a fast flushing flow rate of the flushing solution to the catheter. In these improved catheter flushing systems, a spring biased valve is employed in the flow control device to control a bypass passageway in the device around the capillary flow resistor passageway therein, with the valve being manually operated either by pulling out or pushing in a valve stem, or a valve operating plunger, protruding from the device. Besides their being constituted of many component parts and involving complicated assembly procedures such as renders the device relatively expensive, these valve controlled devices also have certain other deficiencies such as possible accidental undesired opening of the valve or breakage of the valve stem so as to preclude proper operation of the device. Moreover, because of the yieldability of the resilient valves of these devices, the blood pressure pulses to which the valves are exposed during use are dampened at least to some degree by the yielding of the valve so that inaccuracies and loss of fidelity in the recorded pressure pulses will result. In this regard, the extremely sensitive pressure monitoring equipment ordinarily employed with continuous catheter flushing devices will sense and reflect any dampening of the pressure pulses caused by the continuous catheter flushing device.
More recently, as disclosed in U.S. Pat. Nos. 4,192,303: 4,245,636: 4,267,835: and, 4,278,083, improved continuous catheter flushing devices have been developed which, among other things, are of more simplified and less expensive construction and which employ a capillary flow resistor inner tube through which the flushing solution normally passes at a controlled slow rate, and a rubber-like outer sleeve spaced from but elastically gripping, at a region intermediate its ends, around the capillary inner tube to normally seal off the passageway between the capillary tube and the surrounding rubber sleeve. Squeezing of the rubber outer tube adjacent the seal then distorts it out of round so as to break the seal between the capillary tube and the surrounding rubber sleeve, thereby opening the passageway therebetween to form a bypass passageway for fast flow of the flushing solution through the device. On removal of the squeezing force from the rubber sleeve, the latter then contracts to again grip around the capillary inner tube so as to restore the seal therebetween, thereby blocking the bypass passageway to then confine the flow of the flushing solution solely through the capillary tube. However, even with these improved continuous catheter flushing devices employing rubber outer sleeves distortable by squeezing to open a bypass passageway around the capillary inner tube, inaccuracies in the recorded blood pressure pulses have persisted. One reason for these inaccuracies is that the extremely sensitive blood pressure monitoring equipment normally employed at present with intravascular catheter systems will sense and reflect any dampening of the pressure pulses caused by the continuous catheter flushing device, thereby resulting in loss of fidelity in the recorded pressure pulse waveforms. As in the case of continuous catheter flushing systems as previously referred to and employing spring loaded valves normally closing but operable to open a bypass passageway, the improved continuous catheter flushing devices which utilize rubber outer seal-off sleeves around the capillary inner tubes and squeezably distortable to open a bypass passageway, also are subject to some degree of dampening of the blood pressure pulses and resultant loss of fidelity in the recorded pulse waveforms owing to the yieldability of the rubber sleeves to the blood pressure pulses to which they are subjected in the normal use of the catheter flushing device.